Method of controlling the moisture content of leather



A ril 20, 1926.

E. F. HULBERT METHOD OF CONTROLLING THE MOISTURE CONTENT OF LEATHERFiled June 15. 1921 E dwm Iifiulh art; fryf Jame (EJ 55 01 may P l lH II H IHI Milil II.

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Patented Apr. 20, 1926.

UNITED STATES.

. 1,581,803 PATENT OFFICE.

EDWIN F. HULBERT, OF MILWAUKEE, WISCONSIN, AOSIGNOR TO PFISTER &. VOGELLEATHER COMPANY, OF MILWAUKEE, WISCONSIN, A CORPORATION OF WISCONSIN.

METHOD OF CONTROLLING THE MOISTURE CONTENT OF LEATHER.

Application filed June 15, 1921. Serial No. 477,846.

To all whom it may cmwem:

Be it known that I, EDWIN F. HULBERT, a citizen of the United States,residing in the city of Milwaukee, county of Milwaukee,

and State of Wisconsin, have invented a certain new and useful Method ofControlling the Moisture Content of Leather, .of which the following isa specification.

My invention relates primarily to a moisture adjusting treatment ofanimal fibres such as hides and leather in which during the variousoperations it is necessary to vary the moisture content through a widerange. It is of the utmost importance in doing this that the moisturedistribution be uniform throughout the entire body of the materialtreated and not be confined to surface applications or be either greater01 less at thesurface than in the interior of the material.

In the past it has been the practice to remove substantially all of themoisture in drying. This has been done by hanging the leather in openrooms, preferably with the windows open, and raising the temperature bymeans of steam coils or equivalent means. Occasionally an attempt hasbeen made to force the air to circulate by means of fans either locatedin the room or behind the steam coils. After the leather has becomedryit is very hard and before it can be further worked must be partiallymoistened. This is done by dipping the leather in tanks of warm water,with or without other matters in solution therein, draining it, pilingup the leather and perv mitting it to sweat in the moist pack so thatsome of the moisture is driven into the hides, This operation isgenerally called sammying. The leather then is in condition for furtheroperations.

Such methods are crude and unsatisfactory in that there is no propercontrol over either the drying or the remoistening step.

The time required varies greatly dependingupon temperature and humidityconditions, and where the air is made to circulate by fans its dryingefiect is very much limited in that the moisture taken up from the wethides practically saturates the air after the first few feet of traveland for the balance of its travel it exercises no drying effectwhatever. It is only after the first set of leather has been dried thatit be '11s to dry the next setin rotation, etc. he result is that it isvery expensive, wastes time, 1

and does not dry the leather evenly. The leather that is mostadvantageousl placed either with reference to the fans, .t e steamcoils, or other drying means, is dried too rapidly and too hard, whereasthe leather which is less advantageousl placed is dried too slowly andit is impossible to thoroughly control all of the leather and to dry ituniformly as a unit.

The method of moistening back the leather or sammying as practiced iseven more unsatisfactory in that moisture is introduced by means ofsurface application of water in liquid form and the subsequent sweatingprocess so-called does not serve to cause the water to thoroughlypenetrate the leather and do it uniformly. The surface is always moremoist than the interior, and the soft parts like the belly and theflanks, absorb the moisture more rapidly and retain it more readily thando the more dense parts of the hide such as the back and the butt. Theresult is that the leather is un-' evenly dried and unevenly moistened;and not only is there a great loss in time but the resulting product isinferior.

As at present practiced in the case of heavy leathers it isusual to takeas much as 27 0 hours in drying, sammying, and finishing processes onthe leather. Furthermore there is a good deal of non-productive handlingand rehandling. The leather has to be hung and piled, dipped and piled,and moved, all of Wl'llCll is not only expensive but has a deleteriouseffect upon the leather.

Above all the final result is not satisfactory in'that the leather isnot uniformly treated either as between different sides in the same packor as between difierent parts of the same side. Completely dry leatheris hard and horny. When dry fibers are exposed to a moist atmospheretheybegin to take up the water which they have lost, this water beingheld in or on the fiber in the obscure condition known as adsorbed. Thechanges produced are progressive. With a certa n proportion of moisturethe fibers regain their pliability and with a somewhat larger proportionthey begin to swell. The changes are progressive between the completelywet leather and completely dry leather. Air and other gases exposed towater at any given temperature takes up a certain amount of the water asvapor until a certain par- 4 tial pressure or concentration of watervapor in the air is attained. Water, however, held in a state. ofadsorption by leather fiber does not come to the same balance with airas does free waterit resists drying ofi' with somewhat greater force.These principles I apply to the present method.

When wet leather fiber is dried by 3. current of dry air, theevaporation is at first rapid; this representing the evaporation of whatmay be called the free water; the water which is present as actualliquid. Later drying becomes slower as the free Water goes off and theremoval is of absorbed moisture.

In drying leather I have found that, strangely enough, the removal ofthe total quantity of water which I wish to remove goes much morequickly if the initial drying stages be not too rapid; that is, I canget ofi a greater proportion of water in ten hours of total drying if Ido not attempt to remove too much in the first half hour or otherperiod. Wet leather rapidly dried behaves as if the surface pores wereclosed and water sealed in, while with slower drying in the firststages, while the leather is still wet, it behaves as if the pores wereleft open permitting circulation of air through the leather. I find thatit is wholly inadvisable to dry a number of the sides of leather inseries so that the air passes first over one and then over another; thebest conditions are afforded when the air passes the sides of of air toa like moisture content, I establish What may be called a major flow ofnatural air which I warm to a desired degree and then admit a minor flowof similarly'warm air containing a relatively large amount of 7 watervapor. In practice, 'I simply deliver natural air through a fan toheating elements and then admix it with a smaller proportion of hot aircoming from the fan and bypassing through a suitable moistening device.7

Gontrol v of the process then simply means control of' the minor flow ofair.

By my method I am enabled to control the moisture content of leatheruniformly not only as regards each individual hide or side 80 of leatherbut as regards all the sides in the pack that are undergoing treatmentat the same time. Further, I am enabled to very .power, do away with alarge part of the 85 mechanism I am enabled to take a heavy leather sideand condition it so that it is in the best condition for working withina period of 50 to 60 hours, and in the case of light leathers, very muchless.

l. I have found thatgood results cannot be obtained by using rawunconditioned air 95 nor by using water for moistening, that uniformaccurate results can only be obtained in controlling the moisturecontent of leather by use of air at fixed predetermined temperleather,so to speak, in parallel; and .that atures and containing constantpredeterquick drying is effected by performing the first stages ofdrying with a1r already containing considerable proportions of moisture.

The leather should be so hung or arranged as to give the air what may betermed a washing efiect over the entire surface.

Operating in the described manner, that is drying with currents of aircontaining moisture, it is practicable by suitable adjustment ofconditions either to dry the leather down to the point where the exactamount of moisture wanted is left, or to over-step this point and then.bring back the moisture tothe desired point by exposure to a moistatmosphere. By observing these conditions the conversion of wet leatherto leather having the right amount of moisture for convenient working ismuchshorter. Heavy leather can be gotten into good condition for workingwithin a period of 50 to 60 hours as compared with the usual 270 hoursand in case of lightleathers, the time re-. quired is very much less.

emoval of moisture is, of course, much accelerated by heat; but aswarming air decreases the relative humidity, it is necessary, for mypurposes, to add somewhat more water vaporwhen warmed air is used. Inthe most convenient application of my process instead of endeavoring tobring a large body gently downward through a grating, and no dischargedat the bottom. I have illustrated the apparatus for using my method inthe accompanying drawings, in which Fig. 1 i's a view partly in sectionand partly diagram-x matic, taken on the plane of line ,11 of Fig. 2 andFig. 2 is a view similar to Fig. 1 and taken on the plane of line 22thereof.

I have discovered that the ability of the air to extract or depositmoisture in leather is de endent not alone on the temperature or reative humidity but on the combination of the two, and that where Idesireto 'd1- minish the moisture content of leather the best resultsare obtained if the air has a temperature varying from 80 to 95 degreesF. and a moisture content from 35 to 45 per cent relative humidity.Where. it is desired to have the leather take up moisture from the air Ifind best results are obtained by having temperatures of 80 to 85degrees F.

for a period suflicient to dry the leather to the degree desired,varying from 18 to 24 hours or thereabouts, depending upon the weight ofthe leather and the amount of the moisture. Where the leather comes fromthe previous operations and it is'de- I sired to dry it before stuffing,I continue this process until it is in the state generally designated asbone dry. By my method I am enabled to extract practically everyparticle of moisture throughout the entire thickness of theleather, withthe result that the leather will take up a much lar er amount of fats inthe stuffing room an they will more thoroughly penetrate into theleather than is true with the processes heretofore practiced. After theleather has been stufledit is again placed in the chamber and themoisture content lowered. It is not necessary this time toextract somuch of the moisture, but it is desired to bring the leather down to amellow condition. In practicing my method I am enabled to do this in oneprocess, i. e., by extracting only the surface moisture leaving asuflicient amount to make the leather mellow and easily worked. This hasnot been done in practice in the past. It has been dried bone dry andthen dipped and sammied. In this treatment of leather I start in withhigher temperatures and also relative humidity, watching the progress ofthe dehydration and then gradually lowering the temperatures and addingto the relative humidity until the leather is the exact degree ofmoisture content that I desire. If, for instance, I am working withheavy'leathers I would commence using temperatures of 80 to 95 degreeswith a relative humidity of 30 to 35 per cent, subject it to thistemperature for 12 to 14 hours. I would then gradually raise therelative humidity and slowly lower the temperature until I got arelative humidity of 93 to 97 per cent and temperatures,

of from 80 to 85, and continue in this manner for 6 to 10 hours toinsure a uniform distribution of the moisture in the leather..

Whatever method of controlling moisture content is used the surfaces ofthe leather are and must be affected first, and I therefore necessarilyhave to extract slightly more moisture from the surfaces and thenreplace some in order to get a uniform distribution of moisture contentin the final result. By using my moisture carried in a gas I am enabledto do this, which is not the case where it is used in liquid form.Occasionally I desire to use the air to deposit moisture on the surfaceof the leather or part thereof in finely divided liquid form forspecial, purposes and I then make a fog in the bottom of the tunnel ofsuch a density and rising to such height as I may desire. As illustratedin the drawing, the leather is suspended in the chamber or tunnel 5 by asuitable conveyer 6, adjacent hides being spaced to provide vertical airspaces through which the air from a duct 7 passes downwardly to outlets8 located at or near the bottom of the chamber. The exhaust air is drawnthrough the outlets by a suction fan 9 connected therewith by one ormore return ducts 10. If desired for special effects, a fog can be madein the bottom of the chamher or tunnel by admission of cold air, in thedesired volume, through inlets 11 connected by one or moreducts '12 witha cold air supply so as to cool the air below the dew point. 4

Having thus described my invention, I claim:

1. The method of conditioning leather which consists in subjecting it toa stream "of drying air of uniform temperature and moisture contentuntil the leather begins to have a surface hardening and then subjectingthe leather to washing by means of a flow of uniformly moistened aircontaining a relative humidity in excess of 85 per cent until theleather is mellow and of homogeneous moisture content throughout.

2. In the conditioning of leather to bring it to definite moisturecontent, the process which comprises subjecting it to a wash of.

drying air of uniform temperature and moisture content so regulated asto almost entirely remove the free water before re-' moving the adsorbedwater.

3. In the conditioning of leather to bring it to definite moisturecontent, the process which comprises subjecting it to a wash of dryingair of uniform temperature andmoisture content so regulated'as to almostentirely remove the free water before remov-,

ing the adsorbed water and then washing the leather by a stream ofuniformly moistened air of the temperature and moisture content withinthe limitations of the leathers homogeneous absorption and adsorptionlimits until the leather is of homogeneous moisture content.

4. In the conditioning of leather to bring it to definite moisturecontent, the process surface thereof by subjecting the same to anartificially produced fog.

6. In the conditioning of leather to bring it to definite moisturecontent, the process which comprises subjecting both sides of moistleather to a flow of drying air at a temperature of 80 to 85 degrees F.and to ifi per cent relative humidity until its moisture content is lessthan that ultimately desired and then subjecting both sides thereof to aflow of air at a temperature of 80 to 85 degrees F. and 93 to 97 percent relative humidity till its moisture content is at the desiredpoint.

7. In the conditioning oi leather to bring it to definite moisturecontent, the process which comprises subjecting both sides of theleather to a flow of drying air at a temperature of 80 to 85 degrees F.and 30 to per cent relative humidity for a period sufficient to dry theleather to the degree desired.

8. In the conditioning of leather to bring it to definite moisturecontent, the process which comprises subjecting both sides thereof to aflow of air at a temperature of to degrees F. and 93 to 97 per centrelative humidity for a period sutficient to bring the leather to thedesired moistened condition.

9. In the conditioning of heavy leathers to bring them toa' definitemoisture content, the process which comprises subjectin both sides ofthe hide to a flow of drying air at a temperature of 80 to degrees F.With a relative humidity of 30 to 35 per cent for twelve to fourteenhours and then gradually raising the relative humidity of the drying airto 93 to 97 per cent and lowering the temperature of this drying air to80 to 85 degrees F. andcontinuing to subject the hide to air in thelast-named state for six to ten hours to insure uniform distribution ofthe moisture in the leather.

EDIVIN F. HULBERT.

